The present invention is directed to optimizing energy use in a chiller plant transferring heat between cooling towers and a chiller.
A chiller is an air conditioning system which provides a temperature conditioned fluid, usually water, for use in conditioning the air of a load such as a building. Chillers are typically used in large air conditioning systems which centralize the air conditioning requirements for a large building or complex of buildings by using water or a similar fluid as a safe and inexpensive temperature transport medium.
In its operation, the chiller provides conditioned water of a particular temperature for use in cooling air in a building by means of a first water loop. Heat is extracted from the building air, transferred to the water in the water loop, and is returned via the water loop to the chiller which again refrigerates the water to the desired temperature by transferring the heat of the water to the chiller's refrigerant. After the refrigerant is compressed by a compressor or absorbed in an absorber, the heat in the refrigerant is transported to the condenser and heat is transferred to a second water loop. The second water loop transports heat from the condenser of the chiller system to a cooling tower or towers which then transfers the heat from the second water loop to ambient air by direct contact between the ambient air and the water of the second loop.
In the past the water being cooled in the second water loop by the cooling towers has been cooled using one of three strategies. Since the chiller is considered the largest power consumer in the air conditioning system, a first strategy cools the water in the second water loop as cold as possible without regard to the energy used by the cooling tower fan. However, although chillers are still a significant power consumer, they are also the most efficient part of an air conditioning system. Centrifugal chillers such as those sold under the trademark CenTraVac.TM. by The Trane Company, a Division of American Standard Inc., are available at 0.50 kilowatts per ton at ARI rating conditions.
A second tower water temperature control strategy is to produce the warmest possible tower water to obtain a considerable reduction in tower fan energy consumption. However, operating a chiller at elevated tower water temperatures may cause adverse effects over time since the higher than normal pressure differential between the evaporator and condenser places a greater burden on the compressor.
A third strategy for operating a cooling tower is to use the wet bulb temperature plus fixed amount such as five degrees Fahrenheit. However, although tower performance is a function of ambient wet bulb temperature, tower performance is also influenced by the amount of heat being rejected, i.e. the cooling load.
The electrical energy or other energy used by the chiller in cooling the first water loop is a large source of energy usage in the chiller system and an area with potential energy savings. Additionally, since the fan power of the cooling tower fans is proportional to the airflow rate cubed, the energy used by the cooling towers in cooling the second water loop is also another area with potential energy savings by reducing energy usage.
Previously, applicant has attempted to optimize both the amount of energy used by the cooling towers and the amount of energy used by the chillers so as to thereby optimize the overall energy usage of the system. This optimal approach has proven difficult to implement due to the computer intensive calculations required. Each variable must be monitored, and the optimal temperature determined by iteration each time a variable changes.
Johnson Services company, as shown by their U.S. Pat. No. 5,040,377 to Braun et al., uses a near optimal solution where a fan control controls the speed of the cooling tower fans to minimize the total power consumption of the fan and the compressor motors. To the extent that this patent shows a cooling tower/chiller system with a unified control system, this patent is incorporated by reference herein.
Applicant considers that all of these previous approaches can be improved upon.